"Making this measurement has been one of the team's most important scientific goals," says Professor Michele Dougherty, of Imperial College London.

Astronomers have long tried to calculate the length of a Saturn day. William Herschel back in the late 18th century for example, suggested it lasted about 10 hours.

But the nature of Saturn itself makes measurements difficult.

Unlike Earth which has a rocky surface, Saturn is made up mostly of hydrogen and helium gases, which complicates how scientists measure the length of a day.

So rather than take direct measurements, scientists use proxies, or markers, of the planet's rotational period.

In the 1980s, scientists analysed radio signals from Saturn picked up by Voyager to calculate the length of day.

Now, Dougherty and US scientists used an instrument called a magnetometer on the Cassini spacecraft, which measures the planet's magnetic field.

By analysing data collected by Cassini they found a clear period in the planet's magnetic field that they suggest indicates a day lasts 10 hours, 47 minutes and 6 seconds, give or take 40 seconds.

"The period we found from the magnetic field measurements has remained constant since Cassini entered orbit almost two years ago, while radio measurements since the Voyager era have shown large variability," says Dr Giacomo Giampieri, the lead author of the study from NASA's Jet Propulsion Laboratory.

"By measuring the magnetic field over the rest of the mission, we will be able to solve this puzzle."